1. Field of the Invention
This invention relates to the fabrication of integrated circuits with ion beams, and more particularly to direct-write ion beam lithography in which ion implantation is accomplished by focusing an ion beam onto a small spot on a semiconductor substrate and deflecting the beam over the substrate to implant desired patterns, and to systems in which a mask is flooded with ions to form a pattern beam which implants desired regions on an underlying substrate.
2. Description of the Prior Art
Two advanced approaches can be taken to ion implantation which eliminate the use of conventional photolithography and any wafer contacting masks to define the regions to be implanted. The first approach, which may be termed the "flooded" mode of operation, produces a higher throughput of semiconductor wafers with corresponding cost efficiencies and rapid turnaround time. In the flooded mode a mask bearing a circuit pattern is placed in close proximity to, but not touching, a semiconductor wafer upon which a circuit is to be fabricated. A "proximity" mask of this type is to be distinguished from a more conventional "contact" mask, in which a mask is provided directly on the wafer in the form of a pattern of resist or other ion blocking film. The proximity mask is then flooded with a relatively large cross-section ion beam. The mask transmits selected portions of the beam for ion implantation on the underlying substrate, and blocks other potions of the beam. Although this technique is relatively efficient, it is not as flexible as might be desired when it comes to providing particular doping patterns on the wafer. For example, graded doping patterns are desirable when forming the base region of a bipolar transistor. The base contact is heavily doped, whereas the active base region is lightly doped. It is desirable to have a gradual, non-abrupt transition between the doping levels of these two areas. Flooded beam fabrication, however, results in a uniform doping level over those areas of the wafer exposed to the beam, and this us not adaptable to varying the doping level over small portions of the wafer. There are a number of other situations in which a more detailed doping pattern than that generally available with a flooded beam are desirable. In addition, some problems have been encountered in properly aligning the wafer with the mask and beam. One effective technique involves the use of laser beams directed at gratings on the mask and wafer to achieve the desired alignment. However, laser beams represent an entirely different type of radiation, and are difficult to incorporate as part of an ion beam system.
The other lithography approach uses focused ion beams, with the beam reduced to a small spot and deflected over the wafer in an appropriate pattern until a desired doping has been achieved. One such focused beam system that is capable of producing a high resolution, high energy scanning spot is described in co-pending U.S. patent application Ser. No. 482,745, filed Apr. 7, 1983 by J. William Ward et al., entitled "Focused Ion Beam Microfabriation Column" and assigned to the assignee of the present invention. In this system an ion beam emerging from a source is focused to a first crossover in the vicinity of a mass separator, which removes undesired particles from the beam. A first accelerating lens is also positioned in the vicinity of the first beam cross-over to accelerate the beam, while a final accelerating lens is positioned to further accelerate and focus the beam to a small spot on the substrate. A deflector downstream of the final accelerating lens causes the focused beam to follow the desired pattern on the wafer. The energy imparted to the beam by the accelerating lenses is variable, thus permitting the lenses to function as a control over the beam's total energy and depth of implantation.
The focused beam approach permits a higher degree of flexibility in providing detailed implantation patterns on the wafer. However, since the size of the focused beam spot on the wafer is much smaller than the area emcompassed by a flooded beam without a corresponding increase in the current density of the focused beam, it takes much longer to expose a given area on the wafer with a focused beam than with a flooded beam. This greatly reduces the wafer throughput, with a corresponding increase in the cost and time required to produce each individual wafer.